1. Field of the Invention
The present invention relates to an image recording apparatus for recording an image with dots and, more particularly, to a halftone image recording apparatus which is capable of recording an image in halftone.
2. Description of the Prior Art
Conventional dot recording apparatuses include those of wire dot type, electrostatic type, electrophotographic type, ink jet type and so on. As a halftone image forming method with such a dot recording apparatus, a method is known wherein a picture element has a 4.times.4 dot matrix to provide 17 gradation steps. In other words, the picture element has an n.times.m dot matrix to provide (n.multidot.m+1) gradation steps. In accordance with the level of an input signal, one of (n.multidot.m+1) patterns is selected. Dot printing is performed in accordance with the selected pattern. However, when 17 gradation steps are provided with the 4.times.4 dot matrix of this method, the gradation becomes too abrupt for recording an image having a smooth gradation change such as human skin. A pseudooutline is then generated which degrades the image quality. On the other hand, when the number of gradation steps is increased, the area of one picture element is increased, resulting in degradation in resolution. When resolution is degraded, characters, symbols or parts of hairs, for example, are blurred.
As such a halftone image forming method, the dither method is also known. According to this method, the level of the input image signal is systematically converted into binary values through a matrix including sixteen thresholds, from 0 to 15. Such a dither method is introduced, for example, in "Binary Display of Halftone Images According to the Dither Method", Nikkei Electronics, May 1, 1978. However, this dither method is subject to a drawback; the resolution in reading out the image must be improved. A general television picture includes picture information of about 512.times.490 picture elements. Therefore, when the picture information is converted into binary values in correspondence with the original image information and printed accordingly, the gradation is degraded very much and the printing area is extremely decreased (the printing area becomes about 64.times.61.25 mm if printing is performed with a resolution of 8 PEL). When a 4.times.4 dot matrix is used in the density pattern method, the printing size is great (256.times.245 mm) and the pseudooutline cannot be eliminated.
Various other halftone image recording methods are also proposed in addition to this method. However, no such method has been satisfactory.
In other words, when the number of gradation steps is increased, the pseudooutline is formed. On the other hand, when recording is performed by the conventional systematic dither method when the number of input picture elements is limited as in the case of a television picture image, both the halftone image and resolution are degraded. Therefore, a recording apparatus for halftone image recording has been required which solves this problem.